1. Field of the Invention
The present invention relates generally to artificial cardiac pacing, and more particularly to cardiac pacemakers which are selectively adaptable to provide either unipolar or bipolar pacing of the patient's heart.
2. Prior Art
It is, of course, well known that the sinoatrial (S-A) node of the normal mammalian heart acts as the natural pacemaker by which rhythmic electrical excitation is developed and propagated to the atria. In response to this excitation, the atrial chambers contract, thereby pumping blood into the ventricles. The excitation is further propagated through the atrioventricular (A-V) node, which imposes a delay, and then via the conduction system consisting of the bundle of His and Purkinge fibers to the ventricular muscle, thereby causing contraction and the pumping of blood from the ventricles. Disruption of this natural pacing and propagation system occurs as a result of aging and/or disease.
Where normal rate or rhythm is not spontaneously maintained in the heart beat of a human patient, the condition is corrected typically by artifical pacing in which a cardiac pacemaker, selected according to the particular deficiency of the patient, is implanted. In its simplest form, an implantable cardiac pacemaker consists of a pulse generator (or stimulus generator) powered by a self-contained battery pack; and a lead assembly including an electrode adapted to be positioned in stimulating relationship with excitable myocardial tissue either externally (an epicardial electrode) or internally (an endocardial electrode) of the heart, and an insulated electrically conductive lead interconnecting the pulse generator and the tissue-stimulating electrode to deliver the electrical stimuli from the generator to the tissue via an electrical circuit completed by a second electrode and the body tissue and fluids. The electrical stimuli induce desired contractions of the respective chamber at a rate based on the timing of their delivery, the rate being appropriate for a normal sinus rhythm.
The entire lead assembly is often referred to simply as the lead, and the terminology "lead" and "electrode" are sometimes used interchangably, albeit inaccurately. For present purposes, the cardiac tissue-stimulating electrode which is placed in contact with or immediately adjacent the excitable cardiac tissue will be referred to as the stimulating cathodic electrode, or simply the cathode, and the other electrode will be referred to as the anodic electrode, or simply the anode. In fact, however, the coupling may be such that each electrode acts to a certain extent, at different times, as a cathode and an anode. In any event it is well known that activity takes place at both electrodes in the delivery of pacing stimuli. The customary lead choice for the cathode of the implantable cardiac pacemaker is an endocardial lead, because it is readily inserted pervenously into the chamber to be paced. In contrast, an epicardial lead requires thoracic surgery to affix the electrode to the heart. The pulse generator, which is housed with the battery and other circuitry in a conductive case, is typically placed in a subcutaneous pocket formed by an incision in the patient's chest.
Cardiac pacing may be achieved through anodal stimulation rather than cathodal stimulation. However, the stimulation threshold (that is, the minimum electrical impulse necessary to initiate contraction of the excitable cardiac tissue) for anodal stimulation is greater than the threshold achieved using cathodal stimulation. The explanation for this lies in the action of the polarizing force of the stimulating electric field on the ions along membranes of excitable myocardial cells subjected to the field. In essence, the highest current density and current flow exist at the side of each affected cell closest to the cathode. Thus, a cathodal pulse is depolarizing, or stimulating. In the case of anodal stimulation, however, the effect is hyperpolarizing, or nonstimulating. It is essential to the initiation of stimulation that field strength and duration be sufficient to maintain cell depolarization. This reduces the transmembrane potential to a level at which an action potential occurs, in turn spreading depolarization of adjacent cells and the consequent contraction of the tissue. Reduction of transmembrane potential occurs on the side of each affected cell furthest from the anode, and hence at a point of relatively lower field intensity, resulting in a higher threshold for anodal stimulation.
The pacing stimulation provided by the implanted cardiac pacemaker may be unipolar or bipolar, depending on the preference of the physician and the needs of the particular patient. For unipolar stimulation the anode is typically the metal case housing the pulse generator, and as such, is located remote from the heart. With a relatively small number of patients, current distribution to a large electrode in contact with the chest muscle may cause pectoral stimulation as the heart is paced. To avoid this, the conductive surface area of the case may be reduced by coating all but a small portion with non-conductive material, such as paraleen, which is inert to body fluids. This reduction in electrode area in turn reduces the effectiveness of the circuit's ground potential and creates some susceptibility to noise in the circuit.
For those reasons, the implanting physician may prefer to forego the simplicity of unipolar pacing in the case of a particularly sensitive patient, and choose instead to use bipolar pacing. For bipolar stimulation, the cathode and anode are located in relatively close proximity to each other. In a typical arrangement, the cathode is at the distal tip of the endocardial lead and the anode is a ring electrode insulated from the tip and spaced slightly back therefrom, say a half inch or so, on the same lead. Current flow is then between the tip and the ring, rather than the tip and the case. Of course, it is essential, if bipolar pacing is used, that the case be electrically isolated such that it is not a return path for current.
In the past, various schemes have been employed to permit selection of either unipolar or bipolar pacing with an implantable single chamber cardiac pacemaker. One of the earliest techniques employed transformers to isolate the pacing current from the case. However, a transformer fails to provide complete isolation in that the device couples to the conducting medium of the body via a magnetic field, rather than directly. Accordingly, some current will continue to flow through the patient's body during pacing, notwithstanding that its magnitude is smaller than would occur without the transformer. In addition to the possibility of pectoral stimulation, this shunt path for current effectively raises the stimulation threshold and therefore reduces battery life. Moreover, a transformer occupies considerably greater space (and thus increases the size of the implantable device) and is relatively more expensive, compared to other circuit components.
Subsequently, it was proposed that analog or solid state switches be used in techniques for selective unipolar or bipolar pacing and/or sensing. A serious problem encountered with solid state switches is susceptibility to random voltages in the system that can turn the switch on when it should be off, or vice versa. The control voltage for closing or opening the switch (that is, turning it on or off) must be reference to the switch and to the remainder of the circuit to avoid such occurrences.
An example of a prior art single chamber pacemaker configured for selective sensing in either unipolar mode or bipolar mode is found in U.S. Pat. No. 4,402,322. According to that patent, the pacemaker output circuit incorporates tri-state high current buffers and control logic which may be implemented in CMOS process technology, to provide various functions including combinations of unipolar or bipolar sensing. As described in that patent, however, the ring and tip electrodes are free to float in the bipolar mode because they are isolated from the power supply voltages and system ground. Such an arrangement suffers loss of control of the ground reference in the system, and the consequent possibility that a random voltage in the system can create an undesired current path, and hence, incomplete isolation.
These types of problems are magnified when it is sought to provide selective (programmable) bipolar or unipolar pacing for a dual chamber cardiac pacemaker. U.S. Pat. No. 4,462,406 describes an isolation system for a dual chamber pacemaker to reduce the possibility of crosstalk and cross-stimulation between bipolar leads in the atrium and the ventricle, where the leads share a common ground connection. According to that patent, crosstalk is characterized by a crossover of sensed signals in one channel (e.g., the bipolar lead in the atrium) to the other channel (e.g., the bipolar lead in the ventricle), and cross-stimulation is the analogous situation in the pacing mode. The patent describes an arrangement in which the atrial and ventricular leads are multiplexed by toggling a set of semiconductor switches at a high chopping rate and with break-before-make action such that at no instant of time are both channel lead pairs connected to the pacemaker. Inasmuch as one lead pair is always disconnected before the other is connected, there is an instant during each multiplexing cycle in which neither lead pair is connected to the ground, and hence, in which a portion of the system is floating and therefore susceptible to random switching or incomplete isolation. Also, that patent does not address selective unipolar/bipolar pacing in a dual chamber pacemaker.
In U.S. Pat. No. 4,558,702, a dual chamber cardiac pacemaker is provided with an input/output circuit responsive to a command signal to selectively operate in unipolar and bipolar pacing and sensing modes. The circuit of that patent utilizes three P-channel MOS transistor switches to permit selection of any of the four combinations of unipolar and bipolar pacing and sensing, that is, bipolar sensing with unipolar pacing, unipolar sensing with bipolar pacing, unipolar sensing with unipolar pacing, or bipolar sensing with bipolar pacing. However, there is no provision for selective mixed modes of unipolar and bipolar stimulation of the two chambers.